Objective:
The long-term objective is to identify factors that improve fertility by identification of uterine factors that limit pregnancy establishment and maintenance. We propose the central hypothesis that cows differ in uterine receptivity for their ability to become and remain pregnant, even after providing them with a healthy embryo. Identification of these limitations through evaluation of uterine secretions and gene transcription profiles during early pregnancy should lead to identification of associated genomic markers. Furthermore, identification of these genetic markers would allow screening of couples using infertility clinics to identify fertility problems and lead to appropriate recommendations by physicians to rectify the specific fertility problem. The following whole animal to molecular objectives are designed to test the following specific aims during a 5 year study: 1) Develop a unique animal resource to study endometrial receptivity and pregnancy success using natural variation in beef heifer pregnancy establishment, 2) Investigate physiological mechanisms underlying endometrial receptivity and pregnancy success in heifers from this population, 3) Identify genetic markers for endometrial receptivity and uterine competency for pregnancy. In summary, this proposal is aimed at identifying physiological mechanisms and endometrial genes affecting pregnancy establishment and maintenance to develop genetic markers of fertility in cattle and humans.

Approach:
Specific aim 1 and a portion of specific aim 2 will be accomplished at Fort Keogh by ARS employees. Aim 1 will be conducted in year 1 and a portion of year 2 by repeatedly transferring in vitro produced embryos into approximately 300 heifers and determining early pregnancy success (day 45). After 6 replications of transferring embryos, measuring pregnancy success and terminating pregnancies, heifers will be classified as being Superior (> 5 successful pregnancies) or Inferior (< 2 pregnancies) recipients for use in Aims 2 and 3. In aim 2, the physiological mechanisms underlying uterine endometrial receptivity and successful pregnancy will be evaluated. Only studies 2.1 and 2.2 will be conducted at ARS. In study 2.1, serum progesterone concentration will be used to ensure variation of uterine receptivity is not limited by progesterone binding and activation of endometrial progesterone receptors. In study 2.2, the stable plasma metabolite of prostaglandin F2a (PGFM) will be evaluated to estimate the sensitivity of the uterine endometrium for maternal recognition of pregnancy signal. This is accomplished by measuring the prostaglandin response to oxytocin administration that must be blocked to support pregnancy establishment. Heifers that are more sensitive to oxytocin administration might be less likely to become pregnant because of diminished potential for the maternal recognition of pregnancy signal to be perceived. Approximately 50 Superior and 50 Inferior recipient heifers will be identified through the above studies for transport to Washington State University for further studies of differential protein expression (study 2.3) and secretion (study 2.5) by the uterine endometrium, their effects on embryo development (study 2.4), and ultimately variation within the genome of Superior and Inferior recipients as measured by SNP analyses and copy number variants (Aim 3).